A submarine cable system includes a submarine cable (hereinafter, referred to as “cable”), a submarine repeater, a submarine branching device (hereinafter, referred to as “branching unit” (BU)), terminal equipment (hereinafter, referred to as “terminal”), a power supply device, and the like. The submarine repeater amplifies an attenuated signal. The BU branches a signal toward a plurality of points. The terminal transmits and receives various data signals. The power supply device supplies electric power to a submarine device. Note that the power supply device may supply power at a high voltage exceeding several kilovolts. Further, the BU may have a function of switching a power supply path to the submarine device in order to cope with a grounding fault and a fault repair of the cable, and the like.
One example of a submarine branching device having the function of switching a power supply path is disclosed in PTL 1. In a power supply line switching circuit of PTL 1, a power supply line is switched by power supply current. The power supply line switching circuit of PTL 1 includes a relay, a contact, and a resistor. The relay is connected to the power supply line, and operates depending on power supply current. The contact is closed when the relay does not operate, and is opened when the relay operates. The resistor forms a series circuit together with the contact, and the series circuit is connected in parallel with the relay. Note that, when a resistance value of the relay is r, a resistance value of the resistor is R, a value of current flowing through the power supply line immediately before the relay operates is Ia, and a value of current flowing through the power supply line immediately after the relay operates is Ib, r and R are set in such a way as to establish (R/(r+R))·Ia<Ib. As a result of the above-described configuration, a value of current flowing through the relay after the relay operates does not decrease, and thus, the power supply line switching circuit of PTL 1 reliably switches the power supply line without repeating operation and un-operation of the relay.
However, in the power supply line switching circuit of PTL 1, there is a problem that, when the power supply line is switched, large current flows through the relay that connects the power supply line and the ground to each other, and the relay is liable to malfunction.
One example of a technique for suppressing surge current when a power supply path is switched is disclosed in PTL 2. Description is made on a configuration of a submarine branching device of PTL 2, in which attention is paid to a contact of one relay connected to a seawater ground, and a control means for the relay is omitted.
Each of FIGS. 5A and 5B is a schematic diagram illustrating one example of a configuration of the submarine branching device of PTL 2. A BU 200 is connected to a terminal A, a terminal B, and a terminal C via cables 310, 320, and 330, respectively. The BU 200 is connected to a seawater ground (hereinafter, referred to as “sea earth” or “SE”) 340. To the BU 200, electric power is supplied from the terminal A, the terminal B, or the terminal C. To the cables 310 and 320, positive voltage is supplied from the terminal A, and negative voltage is supplied from the terminal B. To the cable 330 between the terminal C and the BU 200, negative voltage is supplied from the terminal C. When a power supply path is switched, the BU 200 is able to ground a partial circuit of the BU 200 to the SE 340 or un-ground a partial circuit of the BU 200 from the SE 340. FIG. 5A illustrates a state where electric power is supplied to the BU 200 by the terminal A and the terminal B. Further, FIG. 5B illustrates a state where electric power is supplied to the BU 200 by the terminal A and the terminal C.
Each of FIGS. 6A and 6B is a diagram for illustrating an operation of a grounding circuit connected to the BU 200. Specifically, FIG. 6A illustrates a state where the BU 200 has been un-grounded, and FIG. 6B illustrates a state where the BU 200 has been grounded.
The grounding circuit 900 includes a contact (RL contact) of a relay (hereinafter, referred to as “RL”) 910, and a resistor 920. One end of the resistor 920 is connected to the SE 340. The other end of the resistor 920 is connected to one end of the RL contact. The other end of the RL contact is connected to the BU 200. The RL contact switches the BU 200 between being grounded and being un-grounded. Note that, the grounding circuit 900 is not included in the BU 200 in FIGS. 6A and 6B in order to make description easily understood, but the grounding circuit 900 may be included in the BU 200. Further, in FIGS. 6A and 6B, a control means for the RL contact is omitted.
Even when the BU 200 is in a state of high electric potential, the grounding circuit 900 is able to suppress, by the resistor 920, surge current flowing through the RL contact when the RL contact is switched and is thereby grounded.